The Lockman Hole

The deepest and longest observation with the ROSAT PSPC was pointed in the
center of the Lockman Hole in the constellation of the Ursa Major (the Big
Dipper). This is the direction of the minimum column density of Galactic
hydrogen (roughly between the pointer stars of the Big Dipper), which allows
the most sensitive search for extragalactic objects. More than 60% of the
extragalactic background above 1 keV can be resolved into over 100 discrete
X-ray sources.